1. Field of the Invention
The present invention relates to a printing apparatus and a print control method, particularly to, for example, a printing apparatus that causes an inkjet printhead to discharge ink droplets to a print medium to print an image and a print control method thereof.
2. Description of the Related Art
An ink droplet discharged from a printhead mounted in an inkjet printing apparatus (to be referred to as a printing apparatus hereinafter) flies in the direction of the resultant vector of the velocity of a carriage that reciprocally scans the printhead and the discharged velocity of the ink. For this reason, if the carriage velocity varies, the landing position of the ink droplet on a print medium shifts, resulting in lower print quality. To prevent this, there is conventionally proposed a technique of adjusting the print timing in accordance with the carriage velocity to improve the ink droplet landing accuracy, thereby improving the image quality.
In a system in which servo velocity control of the carriage velocity is very stable, the image quality is improved by print timing adjustment in a region of the acceleration/deceleration range where the change in the velocity is large. On the other hand, in a constant velocity region, the ink droplet landing accuracy is sometimes improved by suppressing execution of the above-described print timing adjustment. Velocity information used to calculate the shift amount of the print timing is discrete acquired information including quantization errors detected from an encoder signal. Hence, in a case where the carriage velocity is stable, the error factor causes deterioration of the landing accuracy.
To solve this, a method is proposed in which print timing adjustment is executed only when the change in the carriage velocity is larger than a predetermined value. According to this method, control is performed not to automatically execute print timing adjustment in the constant velocity region where the carriage velocity is stable, thereby preventing deterioration of the landing accuracy.
For example, Japanese Patent Laid-Open No. 2005-041028 discloses an arrangement of a related art.
FIG. 11 is a graph for explaining a velocity information acquisition state during carriage acceleration.
Referring to FIG. 11, the ordinate represents a carriage velocity (V), and the abscissa represents a position (X) of the carriage on which the printhead is mounted. The position X is expressed by the distance from the home position of the carriage. A broken line 803 indicates an ideal velocity profile, and • indicates an actual velocity detected at a velocity detection timing ★.
Velocity information detected from a digital signal output from an encoder is obtained every time the carriage moves by a predetermined distance. Hence, the velocity detection timing ★ is obtained at an equal interval with respect to the carriage position. The torque of the carriage motor is adjusted by servo control such that the carriage velocity becomes close to the ideal velocity profile 803.
The actual velocity is shifted from the ideal profile because of a factor such as the state of a load that acts on the carriage driving mechanism. For this reason, in the conventional control method that executes print timing adjustment only when the change in the carriage velocity is larger than a predetermined value, adjustment is executed at timings (A, B, D, F, and G in FIG. 11) at which the velocity difference is larger than a velocity change amount δV to execute the adjustment. To the contrary, adjustment may be not executed at timings (C and E in FIG. 11) at which the velocity difference is small.
Hence, in the conventional method, if the velocity difference is small, print timing adjustment may be not executed, although it is ideal that the adjustment should be executed at each velocity detection timing because the velocity indeed changes during carriage acceleration/deceleration. On the other hand, in the constant velocity region of the carriage movement, print timing adjustment may be executed when a velocity difference of certain level is generated, although satisfactory printing can be performed without executing the print timing adjustment.